Group III Nitride Heterostructure for Optoelectronic Device

ABSTRACT

Heterostructures for use in optoelectronic devices are described. One or more parameters of the heterostructure can be configured to improve the reliability of the corresponding optoelectronic device. The materials used to create the active structure of the device can be considered in configuring various parameters the n-type and/or p-type sides of the heterostructure.

REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 14/493,388, filed 23 Sep. 2014, which claims the benefit ofU.S. Provisional Application No. 61/881,192, filed on 23 Sep. 2013, eachof which is hereby incorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to optoelectronic devices, and moreparticularly, to group III nitride heterostructures for use infabricating optoelectronic devices.

BACKGROUND ART

Development of electronic and optoelectronic devices, such as group IIInitride based light emitting diodes (LEDs), with high efficiency andreliability depends on many factors. Such factors include: the qualityof the semiconductor layers, active layer design, and contact quality.Designing high quality semiconductor layers is especially important fora number of electronic and optoelectronic devices including ultravioletlight emitting diodes (UV LEDs). The quality of a semiconductor layer isdetermined by a number of dislocations in the semiconductor layer aswell as the stresses present in the material. When the stresses becomelarger than threshold stresses, the reliability of a structure can becompromised due to the formation of carrier capturing defects during theoperation of the device. The conducting characteristics of the devicealso can be altered during device operation due to formation of cracksand defects. Furthermore, a resultant junction temperature increase canfurther affect the reliability and efficiency of the device.

To reduce overall stresses in the device and further reduce dislocationdensity, careful selection of epitaxial layers is required.Additionally, carefully selected barriers and quantum wells are neededto produce the target emission wavelength without imposing too muchstresses and strains on the active layer. Furthermore, controllingstresses in the p-type layers are essential in order to ensure thereliability of the device.

Previous approaches have sought to control stresses without sacrificingelectrical properties of the device. In one approach, the semiconductorstructure is grown on a native aluminum nitride (AlN) substrate. Abenefit of growing on an AlN crystal substrate is a small latticemismatch between the substrate and the remaining semiconductor layers.Nevertheless, fabricating on an AlN substrate is difficult andexpensive. Furthermore, only relatively small size AlN substrates can befabricated, resulting in a small device yield.

Currently, a standard approach includes epitaxially growing a group IIInitride semiconductor on a substrate made of sapphire, silicon carbide(SiC), or the like. However, the lattice constant and the coefficient ofthermal expansion are significantly different between the substrate andthe epitaxially grown semiconductor layers. As a result, cracks,dislocations, and pits can develop in the semiconductor layers duringthe epitaxial growth. A quality of a semiconductor layer can be furtheraffected by point defects, compositional inhomogeneities, andinhomogeneities in doping concentration.

To solve this problem, various techniques have been developed tomitigate the effect of the substrate by growing a buffer layer that canabsorb substrate induced stresses, and generally provide a layer whichis closely lattice matched with subsequent epitaxial layers. Forexample, one approach seeks to produce a highly crystalline group IIInitride semiconductor layer, where crack formation is prevented, on asilicon substrate by providing an AlN-based superlattice buffer layerhaving multiple first layers made of Al_(x)Ga_(1-x)N, where the Alcontent x: 0.5<x<1, and multiple second layers made of Al_(y)Ga_(1-y)N,where the Al content y: 0.01<y<0.2, which are alternately stacked,between the silicon substrate and the group III nitride semiconductorlayer.

In another approach for obtaining a highly crystalline group III nitridesemiconductor layer, the group III nitride semiconductor layer is formedon a superlattice composite layer by forming an AlN buffer layer on asilicon substrate and sequentially stacking, on the AlN buffer, acomposition graded layer having a composition graded such that thealuminum content decreases in the crystal growth direction, and asuperlattice composite layer, in which high Al-content layers and lowAl-content layers are alternately stacked.

In still another approach, the AlN buffer layer was grown usingmetalorganic chemical vapor deposition (MOCVD) on a sapphire substrate.Growth conditions were optimized using a two-step growth technique, inwhich the first-step growth was done at a low temperature (1200° C.) andfollowed by the second-step growth at a high temperature (1270° C.). Atthe first-step growth, the substrate was entirely covered bytwo-dimensionally grown AlN by decreasing a V/III ratio to 1.5, althoughmicrocrystalline islands were observed at V/III ratios of 1.2 and 4.0.The approach reportedly yielded an almost pit-free flat surface afterthe second-step growth.

Approaches have sought to control growth of the AlN buffer throughoptimization of MOCVD process. For example, in one approach, the growthcondition of AlN buffer layer was studied to fabricate a high-qualityAlN layer on a sapphire substrate. Trimethylaluminum (TMA) and ammonia(NH₃) were used as precursors for Al and N, respectively. Prior to thegrowth, the substrate was cleaned in an H₂ atmosphere at 1000-1100° C.for ten minutes. The AlN buffer layer was then grown at a V/III ratio of2763 with the growth temperature varied from 800 to 1250° C., with athickness of 5-50 nm. Finally, a 1 μm AlN layer was grown at 1430° C.with a V/III ratio of 584 under a pressure of 30 Torr.

An effect of the substrate on electron mobility can be demonstrated bymeasuring electron mobility in modulation-doped Al_(0.2)Ga_(0.8)N—GaNheterostructures grown on various substrates. For example, FIGS. 1A and1B show electron mobility measurements for sapphire, conducting 6H—SiC,and insulating 4H—SiC substrates as a function of the sheet electrondensity, ns, at the heterointerface. As illustrated by thesemeasurements, a slightly higher electron mobility is obtained on a SiCsubstrate than that on sapphire substrate. The higher electron mobilityis likely attributed to higher quality layers grown on the SiC substrateas compared to sapphire, perhaps due to a lower lattice constantmismatch between AlGaN and SiC.

AlGaN/AlGaN heterostructures as well as AlGaN/GaN heterostructures havevarious traps associated with the dislocations created in the layers.For example, FIG. 2 shows various traps associated with suchheterostructures according to the prior art.

Levels of stresses/strains in a heterostructure, such as an AlGaN/GaNheterostructure, depend on the layer thicknesses. For example, FIG. 3shows relative strain in an Al_(0.25)Ga_(0.75)N layer grown on a thickGaN substrate according to the prior art. When the layer thickness isincreased, the resulting stress is decreased due to formation ofdislocations and other defects (e.g., stress relaxation). For example,prior to the abrupt interface, the strain is high. However, movingfurther away from the abrupt interface, the strain relaxes due to thepresence of dislocations. To a good approximation, the stress dependslinearly and proportionally to the strain within a layer. Consequently,as illustrated in FIG. 3, for an abrupt interface, the stress decreasesrapidly, while the stress decreases more slowly for a graded interface.

A critical thickness of a layer can be defined as the thickness at whichthe dislocations become energetically favorable. For AlGaN layers, thecritical thickness depends on the Al molar ratio. For example, FIG. 4shows the dependence of the critical thickness on the aluminum molarratio of an AlGaN layer in an AlGaN/GaN heterostructure according to theprior art. As illustrated, as the Al molar ratio increases, the stressespresent due to lattice mismatch also increase, which results in adecreasing critical thickness of the AlGaN layer.

SUMMARY OF THE INVENTION

Aspects of the invention provide heterostructures for use inoptoelectronic devices and the resulting optoelectronic devices. One ormore parameters of the heterostructure can be configured to improve thereliability of the corresponding optoelectronic device. The materialsused to create the active structure of the device can be considered inconfiguring various parameters the n-type and/or p-type sides of theheterostructure.

A first aspect of the invention provides a heterostructure comprising: asubstrate; an AlN buffer layer located on the substrate; aAl_(x)Ga_(1-x)N/Al_(x′)Ga_(1-x′)N first superlattice structure locatedon the buffer layer, wherein 0.6<x≦1, 0.1<x′<0.9, and x>x′, and whereineach layer in the first superlattice structure has a thickness less thanor equal to one hundred nanometers; a Al_(y)Ga_(1-y)N/Al_(y′)Ga_(1-y′)Nsecond superlattice structure located on the first superlatticestructure, wherein y′<x′, 0.6<y≦1, 0.1<y′<0.8, and y>y′, and whereineach layer in the second superlattice structure has a thickness lessthan one hundred nanometers; an Al_(z)Ga_(1-z)N n-type layer located onthe second superlattice structure, wherein 0.1<z<0.75 and z<y′; and anAl_(b)Ga_(1-b)N/Al_(q)Ga_(1-q)N active structure, wherein b−q>0.05.

A second aspect of the invention provides a heterostructure comprising:a substrate; a buffer layer located on the substrate, wherein the bufferlayer is formed of a group III nitride material including aluminum; agrading structure located on the buffer layer, wherein the gradingstructure is formed of a group III nitride material having an aluminummolar fraction that decreases from an aluminum molar fraction at abottom heterointerface to an aluminum molar fraction at a topheterointerface; a n-type layer located on the grading structure,wherein the n-type layer is formed of a group III nitride materialincluding aluminum having a molar fraction z, and wherein 0.1<z≦0.9; anactive structure including quantum wells and barriers, wherein thequantum wells are formed of a group III nitride material includingaluminum having a molar fraction q and the barriers are formed of agroup III nitride material including aluminum having a molar fraction b,and wherein b−q>0.05; an electron blocking layer located on the activestructure, wherein the electron blocking layer is formed of a group IIInitride material including aluminum having a molar fraction B, andwherein B is at least 1.05*b; a p-type GaN layer located on the electronblocking layer; and a graded p-type layer located between the electronblocking layer and the GaN layer, wherein the graded p-type layer has analuminum molar fraction that decreases from B at a heterointerfacebetween the electron blocking layer and the graded p-type layer to zeroat a heterointerface between the graded p-type layer and the GaN layer.

A third aspect of the invention provides a method of fabricating adevice, the method comprising: creating a device design for the device,wherein the creating includes configuring a n-type side of aheterostructure for the device based on an active structure in theheterostructure including quantum wells and barriers based on a targetwavelength for the device, wherein the quantum wells are formed of agroup III nitride material including aluminum having a molar fraction qand the barriers are formed of a group III nitride material includingaluminum having a molar fraction b, and wherein b−q>0.05, wherein theconfiguring includes: configuring a grading structure located betweenthe active structure and a buffer layer of the heterostructure, whereinthe grading structure is formed of a group III nitride material havingan aluminum molar fraction that decreases from an aluminum molarfraction at a bottom heterointerface to an aluminum molar fraction at atop heterointerface; and configuring a n-type layer located between thegrading structure and the active structure, wherein the n-type layer isformed of a group III nitride material including aluminum having a molarfraction z selected based on at least one of: b or q; and fabricatingthe device according to the device design.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIGS. 1A and 1B show electron mobility measurements for differentsubstrates as a function of the sheet electron density according to theprior art.

FIG. 2 shows various traps associated with various heterostructuresaccording to the prior art.

FIG. 3 shows relative strain in an Al_(0.25)Ga_(0.75)N layer grown on athick GaN substrate according to the prior art.

FIG. 4 shows the dependence of the critical thickness on the aluminummolar ratio of an AlGaN layer in an AlGaN/GaN heterostructure accordingto the prior art.

FIG. 5 shows an illustrative heterostructure according to an embodiment.

FIG. 6 shows an illustrative heterostructure according to anotherembodiment.

FIG. 7 shows another illustrative heterostructure according to anembodiment.

FIG. 8 shows yet another illustrative heterostructure according to anembodiment.

FIG. 9 shows an illustrative heterostructure including p-type layersaccording to an embodiment.

FIG. 10 shows a schematic structure of an illustrative flip chip lightemitting diode according to an embodiment.

FIG. 11 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide heterostructuresfor use in optoelectronic devices and the resulting optoelectronicdevices. One or more parameters of the heterostructure can be configuredto improve the reliability of the corresponding optoelectronic device.The materials used to create the active structure of the device can beconsidered in configuring various parameters the n-type and/or p-typesides of the heterostructure.

An illustrative embodiment can improve the reliability of a devicethrough simultaneous optimization of several parameters of theheterostructure. These parameters can include: compositional profiles ofthe semiconductor layers; doping profiles of the semiconductor layers;and thicknesses of the semiconductor layers. Additionally, optimizationof strains within the semiconductor layers and resultant polarizationfields also can increase a reliability of the corresponding device.

As used herein, unless otherwise noted, the term “set” means one or more(i.e., at least one) and the phrase “any solution” means any now knownor later developed solution. Additionally, unless otherwise noted, theterm “approximately” and similar terms means within +/−ten percent. Asalso used herein, a layer is a transparent layer when the layer allowsat least ten percent of radiation having a target wavelength, which isradiated at a normal incidence to an interface of the layer, to passthere through. Furthermore, as used herein, a layer is a reflectivelayer when the layer reflects at least ten percent of radiation having atarget wavelength, which is radiated at a normal incidence to aninterface of the layer. In an embodiment, the target wavelength of theradiation corresponds to a wavelength of radiation emitted or sensed(e.g., peak wavelength +/−five nanometers) by an active structure duringoperation of the corresponding device. For a given layer, the wavelengthcan be measured in a material of consideration and can depend on arefractive index of the material.

Turning to the drawings, FIG. 5 shows an illustrative heterostructure 10according to an embodiment. The heterostructure 10 can be configured forlight emission and/or light sensing. To this extent, the heterostructure10 can be used in fabricating an optoelectronic device, such as aconventional or super luminescent light emitting diode (LED), a lightemitting laser, a laser diode, a light sensor, an ultraviolet sensor, aphotodetector, a photodiode, an avalanche diode, and/or the like. In anillustrative embodiment, the optoelectronic device is configured tooperate as an emitting device, such as a light emitting diode (LED). Inthis case, during operation of the optoelectronic device, application ofa bias comparable to the band gap results in the emission ofelectromagnetic radiation from an active structure 22 of theheterostructure 10. The electromagnetic radiation emitted by theheterostructure 10 can have a peak wavelength within any range ofwavelengths, including visible light, ultraviolet radiation, deepultraviolet radiation, infrared light, and/or the like. In anembodiment, the heterostructure 10 is configured to emit radiationhaving a dominant wavelength within the ultraviolet range ofwavelengths. In a more specific embodiment, the dominant wavelength iswithin a range of wavelengths between approximately 210 andapproximately 350 nanometers.

The heterostructure 10 includes a substrate 12, a buffer layer 14adjacent to the substrate 12, a first superlattice structure 16 adjacentto the buffer layer 14, a second superlattice structure 18 adjacent tothe first superlattice structure 16, an n-type layer 20 (e.g., acladding layer, electron supply layer, contact layer, and/or the like)adjacent to the second superlattice structure 18, and an activestructure 22 adjacent to the n-type layer 20. In an embodiment, eachsubsequent structure/layer is epitaxially grown on a previousstructure/layer using any solution. The substrate 12 can be sapphire,silicon carbide (SiC), silicon (Si), GaN, AlGaN, AlON, LiGaO₂, oranother suitable material, and the buffer layer 14 can be composed ofAlN, an AlGaN/AlN superlattice, and/or the like.

In an illustrative embodiment, the heterostructure 10 is a group III-Vmaterials based heterostructure, in which some or all of the variouslayers/structures are formed of elements selected from the group III-Vmaterials system. In a still more particular illustrative embodiment,the various layers of the heterostructure 10 are formed of group IIInitride based materials. Group III nitride materials comprise one ormore group III elements (e.g., boron (B), aluminum (Al), gallium (Ga),and indium (In)) and nitrogen (N), such that B_(W)Al_(x)Ga_(Y)In_(Z)N,where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. The molar fractions given by W, X,Y, and Z can vary between the various layers of the heterostructure 10.Illustrative group III nitride materials include binary, ternary andquaternary alloys such as, AlN, GaN, InN, BN, AlGaN, AlInN, AlBN,AlGaInN, AlGaBN, AlInBN, and AlGaInBN with any molar fraction of groupIII elements. Illustrative aspects of the invention are furtherdescribed in conjunction with ternary AlGaN layers. However, it isunderstood that these layers are only illustrative of the nitridematerials that can be utilized. For example, in other embodiments, oneor more of the semiconductor layers can include indium, thereby forminga quaternary AlInGaN layer. To this extent, quantum wells in an activestructure described herein can contain indium, e.g., at least onepercent indium in a more particular embodiment.

The heterostructure 10 can be configured for emission or sensingelectromagnetic radiation of a target wavelength. To this extent, theactive structure 22 can be formed of materials suitable for generatingand/or sensing electromagnetic radiation of the target wavelength. Basedon the materials utilized in the active structure 22, the remainder ofthe heterostructure 10 can be configured (e.g., optimized) to, forexample, control stresses within the active structure 22, polarizationfields, conductivity of the layers, mitigation of defects, and/or thelike, which in turn can lead to improved reliability and/or quantumefficiency for the heterostructure 10 and corresponding device.

Configuration of the remainder of the heterostructure 10 can includeselection and/or adjustment of one or more of various attributes of thelayers/structures in the heterostructure 10, inclusion or removal of oneor more layers/structures in the heterostructure 10, a change in orderof the layers/structures in the heterostructure 10, and/or the like.Illustrative layer/structure attributes include, for example, adjustmentof: an aluminum molar fraction in a material of the layer/structure; athickness of a layer/structure; one or more growth conditions of thelayer/structure (e.g., temperature, pressure, V/III ratio, etc.); adoping concentration of the layer/structure; and/or the like.

For a group III nitride heterostructure 10, the buffer layer 14 can beepitaxially grown over the substrate 12. For example, the buffer layer14 can be formed of aluminum nitride (AlN). Alternatively, the bufferlayer 14 can be formed of AlGaN having an aluminum molar fractionbetween 0.3 and 1. The buffer layer 14 can be grown to a thicknessconfigured to provide a sufficient amount of stress relief and/ordislocation reduction. In an illustrative embodiment, the buffer layer14 can have a thickness greater than 0.1 microns. Additionally, thethickness of the buffer layer 14 can be less than or equal to onehundred microns to minimize the development of cracks within the bufferlayer 14. For example, the buffer layer 14 can have a thickness between5 Angstroms and 100 microns.

The first superlattice structure 16 can be grown (e.g., deposited) overthe buffer layer 14. The first superlattice structure 16 can includemultiple periods 16 ₁ . . . 16 _(n), each of which includes two layers16A, 16B. In an illustrative embodiment, for each period 16 ₁ . . . 16_(n), the layer 16A is formed of Al_(x)Ga_(1-x)N and the layer 16B isformed of Al_(x′)Ga_(1-x′)N. The layer 16B can have an aluminum molarfraction x′ in a range 0.1<x′<0.8. The layer 16A can have a higheraluminum molar fraction than that of the layer 16B (e.g., x>x′).However, it is understood that this is only illustrative and otherembodiments are possible. For example, the first superlattice structure16 could include periods formed of any number of two or more layers 16A,16B. Additionally, the layer 16A of the periods 16 ₁ . . . 16 _(n) canbe formed of AlN. However, when the layer 16A includes gallium, adifference between x and x′ can be less than 0.5. A thickness of eachlayer 16A, 16B can be between 1 nanometer and 100 nanometers, and thefirst superlattice structure 16 can include between ten and one hundredperiods. In an illustrative embodiment, the first superlattice structure16 includes layers 16A, 16B having thicknesses of approximately thirtynanometers (within a range of 15-50 nanometers) and a total ofapproximately forty periods. In another embodiment, the layers 16A canhave thicknesses in the range of 1-50 nanometers, and the layers 16B canhave thicknesses in the range of 5-100 nanometers.

The second superlattice structure 18 can be grown (e.g., deposited) overthe first superlattice structure 16. The second superlattice structure18 can include multiple periods 18 ₁ . . . 18 _(m), each of whichincludes two layers 18A, 18B. In an illustrative embodiment, for eachperiod 18 ₁ . . . 18 _(m), the layer 18A is formed of Al_(y)Ga_(1-y)Nand the layer 18B is formed of Al_(y′)Ga_(1-y′)N. The layer 18B can havean aluminum molar fraction y′ in a range 0.1<y′<0.65. The layer 18A canhave a higher aluminum molar fraction than that of the layer 18B (e.g.,y>y′). However, it is understood that this is only illustrative andother embodiments are possible. For example, the second superlatticestructure 18 could include periods formed of any number of two or morelayers 18A, 18B. Additionally, the layer 18A of the periods 18 ₁ . . .18 _(m) can be formed of AlN. However, when the layer 18A includesgallium, a difference between y and y′ can be less than 0.5. A molarfraction y′ for the layer 18B can be configured based on the molarfraction x′ for the layer 16B in the first superlattice structure 16.For example, y′ can be less than x′. In an embodiment, y′ is at least0.05 lower than x′. A thickness of each layer 18A, 18B can be between 1nanometer and 100 nanometers, and the second superlattice structure 18can include between ten and one hundred periods. In an illustrativeembodiment, the second superlattice structure 18 includes layers 18A,18B having thicknesses of approximately thirty nanometers (within arange of 15-50 nanometers) and a total of approximately forty periods.In another embodiment, the layers 16A can have thicknesses in the rangeof 1-50 nanometers, and the layers 16B can have thicknesses in the rangeof 5-100 nanometers.

The n-type layer 20 can be grown (e.g., deposited) over the secondsuperlattice structure 18. The n-type layer 20 can be formed ofAl_(z)Ga_(1-z)N, where 0.1<z<0.7 and have a thickness between 0.1microns and fifty microns. The molar fraction z of the n-type layer 20can be selected based on the molar fraction y′ for the layer 18B in thesecond superlattice structure 18. For example, the molar fraction z canbe selected to be less than the molar fraction y′. Additionally, then-type layer 20 can comprise an n-type doping (e.g., using siliconatoms) with a doping concentration on the order of 10¹⁸ dopants per cm³.

The active structure 22 can be grown (e.g., deposited) over the n-typelayer 20. The active structure can be undoped or comprise an n-typedoping (e.g., using silicon atoms) with a doping concentration on theorder of 10¹⁸ dopants per cm³. As illustrated by a band gap diagramshown in the inset of FIG. 5, the active structure 22 can include amultiple quantum well structure formed of barriers 24 comprisingAl_(b)Ga_(1-b)N and quantum wells 26 formed of Al_(q)Ga_(1-q)N, whereb−q>0.05. The barriers 24 in the active structure 22 can havethicknesses in a range of 5 nanometers to 25 nanometers (e.g., about tennanometers), while the quantum wells 26 can have thicknesses in a rangeof 1 nanometer to 5 nanometers (e.g., about two nanometers). However, itis understood that the actual thicknesses of the barriers 24 and quantumwells 26 can vary between fifty and one hundred percent around thesethicknesses.

The barriers 24 in the active structure 22 have a higher aluminum molarfraction b than the aluminum molar fraction q of the quantum wells 26.The quantum well aluminum molar fraction q, the thicknesses of thequantum wells 26, as well as the thicknesses and aluminum molarconcentrations b of the barriers 24 can be chosen such that the activestructure 22 emits electromagnetic radiation having a target wavelength,while maximizing radiative recombination and the injection efficiency ofthe heterostructure 10 using any solution.

The aluminum molar fractions x, x′, y, y′, z, b, and q can be configuredbased on a target wavelength for the active structure 22. In anembodiment, the heterostructure 10 is configured for inclusion in anoptoelectronic device having a target wavelength in the ultravioletrange of wavelengths. In this case, the active structure 22 can beconfigured, e.g., by adjusting the aluminum molar fractions b, q to emitor sense electromagnetic radiation of the target wavelength using anysolution. In general, for smaller target wavelengths, the aluminum molarfractions b, q, increase.

Based on the aluminum molar fractions b, q, the aluminum molar fractionsx, x′, y, y′, and z can be configured to control the stresses within theactive layer 22, as well as the polarization fields, and/or the like.For example, the active structure 22 can be configured to emitelectromagnetic radiation having a peak wavelength between 260nanometers and 300 nanometers. In this case, the aluminum molar fractionb can be in a range of 0.4<b<0.7 while the aluminum molar fraction q canbe in a range of 0.2<q<0.6. Furthermore, the aluminum molar fractionsx′, y′, and z can be within the ranges: 0.4<z<0.75; 0.5<y′<0.8; and0.6<x′<0.9, where z<y′<x′. As described herein, the aluminum molarfractions x, y can be one, in which case the corresponding layers areAlN, or can be higher than the corresponding x′, y′ molar fractions byless than 0.5. Furthermore, the buffer layer 14 can comprise an AlGaNlayer having an aluminum molar fraction between 0.7 and 1.

Due to a high stress field within the active structure 22 of theheterostructure 10, it is desirable to keep the band gap of the quantumwells 26 significantly lower than that of the target wavelength. Theband gap of the quantum wells 26 can be reduced by reducing the aluminummolar fraction q of the quantum wells 26. To this extent, an activestructure 22 configured to emit electromagnetic radiation having a peakwavelength between 300 nanometers and 360 nanometers typically has lowerb, q values than those utilized for the 260-300 nanometer range. Forexample, the aluminum molar fraction b can be in a range of 0.1<b<0.6while the aluminum molar fraction q can be in a range of 0<q<0.35. In amore particular example for emission optimized around 310 nanometers,the aluminum molar fraction q of the quantum wells 26 in the activestructure 22 can be in a range of 0.15≦q≦0.25 and the aluminum molarfraction b for the barriers 24 can be in a range of 0.31≦b≦0.5, whereb−q>0.05 and b>q>0. To this extent, the composition of the n-type layer20 can have a lower aluminum molar fraction z than that utilized for the260-300 nanometer range of wavelengths. Similarly, the aluminum molarfractions x′, y′ also can be lower than those used for the 260-300nanometer range of wavelengths. In general, for operation in the 300-360nanometer range (more particularly in the 310-320 nanometer range), thealuminum molar fraction z can be about 5-50% (5-30% in a more particularembodiment) less than the aluminum molar fraction z used for acorresponding 260-300 nanometer structure and the aluminum molarfractions x′ and y′, can be about 10-40% (10-30% in a more particularembodiment) less than the aluminum molar fractions x′, y′ used for thecorresponding 260-300 nanometer structure. To this extent, anillustrative embodiment of the aluminum molar fractions x′, y′, and zused for a 300-360 nanometer heterostructure 10 can correspond to theranges: 0.25<z<0.5; 0.45<y′<0.65; and 0.6<x′<0.8, where z<y′<x′.Furthermore, the buffer layer 14 can comprise an AlGaN layer having analuminum molar fraction between 0.3 and 0.8.

An active structure 22 configured to emit electromagnetic radiationhaving a peak wavelength between 230 nanometers and 260 nanometerstypically has higher b, q values than those required for the 260-300nanometer range. For example, the aluminum molar fraction q for thequantum wells 26 can be in a range of 0.45<q<0.75 and the aluminum molarfraction b for the barriers can be in a range of 0.55<b<0.9, whereb-q>0.05 and b>q>0.2. To this extent, the composition of the n-typelayer 20 can have a higher aluminum molar fraction z than that utilizedfor the 260-300 nanometer range of wavelengths. For example, thealuminum molar fraction z can be in a range of 0.6≦z<0.9, where z>q. Inthis case, the heterostructure 10 can be formed without one or both ofthe superlattice structures 16, 18 as stress relief between an AlNbuffer layer 14 and the n-type layer 20 may not be required.

Similarly, the aluminum molar fractions x′, y′ also can be lower thanthose used for the 260-300 nanometer range of wavelengths. In general,for operation in the 300-360 nanometer range (more particularly in the310-320 nanometer range), the aluminum molar fraction z can be about5-50% (5-30% in a more particular embodiment) less than the aluminummolar fraction z used for a corresponding 260-300 nanometer structureand the aluminum molar fractions x′ and y′, can be about 10-40% (10-30%in a more particular embodiment) less than the aluminum molar fractionsx′, y′ used for the corresponding 260-300 nanometer structure. To thisextent, an illustrative embodiment of the aluminum molar fractions x′,y′, and z use for a 300-360 nanometer heterostructure 10 can correspondto the ranges: 0.25<z<0.5; 0.45<y′<0.65; and 0.6<x′<0.8, where z<y′<x′.

In an embodiment, it is desirable that the structures/layers 14, 16, 18,20 be transparent to the electromagnetic radiation having a targetwavelength for the active structure 22. In this case, a band gap of thebuffer layer 14, an average band gap of each of the superlatticestructures 16, 18, and a band gap of the n-type layer 20 can be similarto or higher than that of the conduction/valence energy level separationwithin the quantum wells 26 of the active structure 22. Thisconfiguration can result in transparency of the structures/layers 14,16, 18, 20 located between the substrate 12 and the active structure 22and avoid internal absorption of electromagnetic radiation having thetarget wavelength.

One or more additional attributes of the growth of the structures/layers14, 16, 18, 20 and/or the resulting structures/layers 14, 16, 18, 20 canbe configured based on stresses developed in the correspondingstructure. For example, one or more of: the thickness, composition,and/or growth conditions of a semiconductor layer can be configured sothat the stresses do not exceed the threshold stresses developed due tolattice mismatch, thermal stresses, and stresses resulting duringformation of the semiconductor layer (e.g., including stresses due tocoalescence of semiconductor grains or islands formed during the growthprocess). In an embodiment, stresses in an overall semiconductor filmare evaluated by analyzing the bowing of the substrate 12. For example,the Stoney formula can be used to link the bowing and the stresses inthe semiconductor layers, wherein the Stoney formula is given by:

${\sigma = \frac{E_{s}h_{s}^{2}k}{6{h_{f}\left( {1 - v_{s}} \right)}}},$

where σ is the stress in the semiconductor composite film; E_(s) isYoung's modulus of the substrate 12; h_(s) is the substrate 12thickness; k is the substrate 12 curvature; h_(f) is the semiconductorcomposite film thickness; and v_(s) is the substrate 12 Poisson ratio.The Stoney formula involves assumptions that the thickness of thesemiconductor composite film is significantly smaller than the thicknessof the substrate 12 and that elastic isotropic conditions accuratelydescribe the conditions in the substrate-film system.

It is understood that the heterostructure 10 is only illustrative. Tothis extent, a heterostructure can include one or more additionallayers/structures. Similarly, it is understood that a heterostructuredescribed herein can be implemented without one or more of thelayers/structures, regardless of the target wavelength. For example, anembodiment of the heterostructure can include only one or neither of thesuperlattice structures 16, 18. To this extent, when the heterostructuredoes not include one or both superlattice structures 16, 18, a gradedsemiconductor layer can be included between the buffer layer 14 and then-type layer 20, for which the composition slowly changes along thethickness from the layer, e.g., from a composition comparable to thebuffer layer 14 to a composition comparable to the n-type layer 20.Furthermore, it is understood that one or more of the layers/sublayersdescribed herein can include one or more additional attributes. Forexample, a layer/sublayer can be formed such that at least a portion ofa surface of the layer/sublayer is textured (e.g., using etching,sputtering, molecular beam epitaxy, and/or the like). The texturing canbe configured to promote adhesion, reduce stress, increase lightextraction, and/or the like.

FIG. 6 shows an illustrative heterostructure 30 according to anotherembodiment. In this case, the heterostructure 30 includes a gradingstructure 32 located between the second superlattice structure 18 andthe n-type layer 20. The grading structure 32 can be formed ofAl_(g(h))Ga_(1-g(h))N, where the aluminum molar fraction g is a functionof a height coordinate h within the grading structure 32. In anembodiment, the aluminum molar fraction g has a grading such thatg(h=h0)=y′ and g(h=h1)=z, where h0 is the height coordinatecorresponding to the heterointerface between the grading structure 32and a top layer of the second superlattice structure 18 and h1 is theheight coordinate corresponding to the heterointerface between thegrading structure 32 and the n-type layer 20. In a more particularembodiment, the grading is a linear grading and can be calculated asg(h)=y′+(h−h0)·(z·y′)/(h1·h0). A thickness d of the grading structure 32can vary from 5 nanometers to 1000 nanometers depending on the targetwavelength of the electromagnetic radiation and can be either doped orundoped. In a more particular embodiment, the thickness d can be in arange between 150 nanometers and 280 nanometers. When doped, the gradingstructure 32 can have an n-type doping concentration (e.g., usingsilicon atoms) on the order of 10¹⁸ dopants per cm³.

It is understood that the heterostructure 30 is only illustrative. Forexample, in another embodiment, the grading structure 32 can be locatedon a different underlying layer, such as the buffer layer 14 (e.g., whenneither superlattice structure 16, 18 is included in theheterostructure). To this extent, the grading structure 32 can have analuminum molar fraction g(h=h0) that is approximately equal to thealuminum molar fraction of the top surface of the underlying layer(e.g., the buffer layer 14) on which the grading structure 32 is formed(e.g., epitaxially grown). Additionally, it is understood that thelinear grading described herein is only illustrative of various gradingapproaches. For example, in an embodiment, the grading can be adjustedin a series of steps as the grading structure 32 is grown. Furthermore,in another embodiment, the grading structure 32 can be an undoped (e.g.,unintentionally doped) layer having an aluminum molar fraction that isless than or equal to the aluminum molar fraction of the underlyinglayer (e.g., y′) and greater than or equal to the aluminum molarfraction z of the n-type layer 20.

FIG. 7 shows another illustrative heterostructure 40 according to anembodiment. In this case, the heterostructure 40 includes severaltensile/compressive superlattices (TCSLs) 42A-42D located in theheterostructure 40 between the buffer layer 14 and the n-type layer 20.It is understood that while the heterostructure 40 is shown includingfour TCSLs 42A-42D, a heterostructure 40 can include any combination ofany number of one or more TCSLs 42A-42D within the heterostructure 40.For example, a heterostructure 40 can include: only TCSL 42A; TCSLs 42A,42C; and/or the like.

Each TCSL 42A-42D comprises alternating compressive or tensile layers,which can be configured to reduce the stress in the heterostructure 40,bend threading dislocations, and/or the like. Formation of such layers42A-42D can be controlled by growth parameters such as the V/III ratio,temperature, pressure, and/or the like, which can result in changes tothe lattice parameter for the material. Additional discussion regardingthe formation and use of TCSLs 42A-42D is included in U.S. patentapplication Ser. No. 13/692,191, titled “Epitaxy Technique for GrowingSemiconductor Compounds,” which was filed on 3 Dec. 2012, and which ishereby incorporated by reference. As described therein, tensile andcompressive layers can be obtained by changing the V/III ratio of theprecursors during growth, such as metalorganic chemical vapor deposition(MOCVD) growth.

In an embodiment, a TCSL 42A-42D (e.g., TCSL 42D) includes alternatingAl_(u)Ga_(1-u)N tensile layers, where 0.3<u<1, and Al_(t)Ga_(1-t)Ncompressive layers, where 0.1<t<1, which can be epitaxially grown usingany solution. The tensile layers can have a lattice mismatch with thecompressive layers of at least 0.1%. Each layer in the TCSL 42A-42D canhave a thickness between 5 nanometers and 5000 nanometers. In a moreparticular embodiment, a TCSL 42A-42D, such as TCSL 42D, includes layershaving thicknesses between 5 nanometers and 30 nanometers and 0.3<u,t<1.When a heterostructure includes one or more TCSLs 42C, 42D locatedimmediately adjacent to the grading structure 32, it is understood thatthe grading structure 32 can have a varying aluminum content that isselected based on the corresponding immediately adjacent sublayer of thecorresponding TCSL 42C, 42D.

FIG. 8 shows yet another illustrative heterostructure 50 according to anembodiment. In this case, the heterostructure 50 includes a single TCSL42 located between the grading structure 32 and a compound n-type layer20. The compound n-type layer 20 is formed of four n-type sublayers52A-52D. The sublayers 52A-52D can be configured to provide a varyingaluminum molar fraction and/or doping concentration along the height ofthe n-type layer 20.

In an illustrative embodiment, such as when the target wavelength isbetween 260-300 nanometers, the first n-type sublayer 52A comprises anAl_(z1)Ga_(1-z1)N layer having a thickness in a range of 0.1 to 3microns, an aluminum molar fraction z1 in a range of 0.5≦z1≦0.7, and ann-type doping concentration on the order of 10¹⁸ dopants per cm³ (e.g.,in a range of 5×10¹⁷ to 5×10¹⁸ dopants per cm³); the second n-typesublayer 52B comprises an Al_(z2)Ga_(1-z2)N layer having a thickness ina range of 0.1 to 0.4 microns, an aluminum molar fraction z2 comparableto or the same as z1, and an n-type doping concentration at least tenpercent higher than the doping concentration of the first n-typesublayer 52A; the third n-type sublayer 52C comprises anAl_(z3)Ga_(1-z3)N layer having a thickness in a range of 0.1 to 0.4microns, an aluminum molar fraction z3 comparable to or the same as z2,and an n-type doping concentration on the order of 10¹⁸ dopants per cm³;and the fourth n-type sublayer 52D comprises an Al_(z4)Ga_(1-z4)N layerhaving a thickness in a range of 0.05 to 0.4 microns, an aluminum molarfraction z4 approximately ten percent smaller than z3, and an n-typedoping concentration at least ten percent smaller (twenty percentsmaller in a more particular embodiment) than the doping concentrationof the third n-type sublayer 52C.

It is understood that four sublayers 52A-52D is only illustrative, andthe n-type layer 20 can be formed of any number of sublayers. Forexample, in another illustrative embodiment, such as when the targetwavelength is between 300-360 nanometers, the n-type layer 20 caninclude five sublayers. In a more particular illustrative embodiment,the first n-type sublayer comprises an undoped Al_(z1)Ga_(1-z1)N layerhaving a thickness in a range of 0.1 to 3 microns and an aluminum molarfraction z1 in a range of 0.1≦z1≦0.6; the second n-type sublayercomprises an Al_(z2)Ga_(1-z2)N layer having a thickness in a range of0.1 to 3 microns, an aluminum molar fraction z2 comparable to or thesame as z1, and an n-type doping concentration on the order of 10¹⁸dopants per cm³ (e.g., in a range of 5×10¹⁷ to 5×10¹⁸ dopants per cm³);the third n-type sublayer comprises an Al_(z3)Ga_(1-z3)N layer having athickness in a range of 0.1 to 0.4 microns, an aluminum molar fractionz3 comparable to or the same as z2, and an n-type doping concentrationat least ten percent higher than the doping concentration of the secondn-type sublayer; the fourth n-type sublayer comprises anAl_(z4)Ga_(1-z4)N layer having a thickness in a range of 0.1 to 0.4microns, an aluminum molar fraction z4 comparable to or the same as z3,and an n-type doping concentration comparable to the dopingconcentration of the third n-type sublayer; and the fifth n-typesublayer comprises an Al_(z5)Ga_(1-z5)N layer having a thickness in arange of 0.1 to 0.3 microns, an aluminum molar fraction z5 approximatelyten percent smaller than z4, and an n-type doping concentration at leastten percent smaller than the doping concentration of the fourth n-typesublayer.

A heterostructure described herein can include one or more layers on ap-type side of the active structure 22, which are configured to improveone or more aspects of the reliability and/or operation of a deviceincluding the heterostructure. For example, FIG. 9 shows an illustrativeheterostructure 60 including p-type layers 62, 64, 66 according to anembodiment. While the heterostructure 60 is shown including three p-typelayers 62, 64, 66, it is understood that embodiments of aheterostructure can include any combination of one or more p-type layers62, 64, 66.

The heterostructure 60 can include an electron blocking layer 62 locatedadjacent to the p-type side of the active structure 22 (e.g.,epitaxially grown thereon). The electron blocking layer 62 can beconfigured to improve injection efficiency (e.g., a total recombinationcurrent relative to the total current in the heterostructure 60) of theheterostructure 60. In an embodiment, the aluminum molar fraction of theelectron blocking layer 62 is at least five percent (ten percent in amore particular embodiment) larger than the barrier aluminum molarfraction b. To this extent, the aluminum molar fraction of the electronblocking layer 62 can be in a range between 0.2 and 1. In a moreparticular embodiment, the electron blocking layer 62 comprises asemiconductor layer having a high aluminum content, e.g., an aluminummolar fraction in a range of 0.5 to 0.9, which is designed to blockelectrons from injection into the p-type layer 64. The electron blockinglayer 62 can have a thickness in the range of 5 nanometers to 100nanometers (10 nanometers to 50 nanometers in a more specificembodiment) and a p-type doping concentration in the range of 10¹⁶ to10¹⁸ dopants per cm³.

In an alternative embodiment, the electron blocking layer 62 is formedof an Al_(s1)Ga_(1-s1)N/Al_(s2)Ga_(1-s2)N superlattice, where 0.2<s1<0.8and 9<s2<0.5. Each layer of the superlattice can have a thickness in therange of 0.5 nanometers to 5 nanometers, and a p-type dopingconcentration in the range of 10¹⁷ to 10¹⁹ dopants per cm³.

The heterostructure 60 also can include a graded p-type layer 64, whichcan be epitaxially grown over the electron blocking layer 62. The gradedp-type layer 64 can be formed of Al_(p)Ga_(1-p)N, where 0≦p≦0.9, have athickness in a range of 10 nanometers to 500 nanometers, and have ap-type doping concentration in a range of 0 to 10¹⁹ dopants per cm³. Thegraded p-type layer 64 can have a graded aluminum molar fraction p. Forexample, the aluminum molar fraction p can vary from the aluminum molarfraction of the electron blocking layer 62 at the heterointerfacebetween the electron blocking layer 62 and the graded p-type layer 64 toan aluminum molar fraction of the p-type layer 66 (e.g., zero) at theheterointerface between the graded p-type layer 64 and the p-type layer66. In a more particular embodiment, the aluminum molar fraction p has alinear grading along a height of the graded p-type layer 64.

Similarly, the graded p-type layer 64 can have a graded dopingconcentration. For example, the doping concentration can vary from twicethe doping concentration of the electron blocking layer 62 at theheterointerface between the electron blocking layer 62 and the gradedp-type layer 64 to zero at the heterointerface between the graded p-typelayer 64 and the p-type layer 66. In a more particular embodiment, thedoping concentration has a linear grading along a height of the gradedp-type layer 64. Alternatively, the doping concentration and/or aluminummolar fraction p can be adjusted in a series of steps as the gradedp-type layer 64 is grown. In another embodiment, the aluminum molarfraction p is graded in a first portion of the graded p-type layer 64while the doping concentration remains substantially constant, and thedoping concentration is graded in a second portion of the graded p-typelayer 64 while the aluminum molar fraction p remains substantiallyconstant.

It is understood that the graded p-type layer 64 is only illustrative.For example, in another embodiment, the graded p-type layer 64 can havea constant composition and/or a constant doping concentration. Forexample, such a layer can be formed of AlGaN material having an aluminummolar fraction up to 0.6 (0.4 in a more particular embodiment), athickness in a range of 1 nanometer to 500 nanometers, and a dopingconcentration in a range of 1×10¹⁷ dopants per cm³ and 1×10¹⁹ dopantsper cm³.

The p-type layer 66 (e.g., a cladding layer, hole supply layer, contactlayer, and/or the like) can be formed of GaN (having an aluminum molarfraction of zero) and can have a doping concentration in a range of1×10¹⁸ dopants per cm³ and 1×10²⁰ dopants per cm³. As illustrated, anembodiment of the p-type layer 66 can be formed of a series of sublayers68A-68C. While three sublayers 68A-68C are shown, it is understood thatany number of two or more sublayers 68A-68C can be utilized. Regardless,in an illustrative embodiment, the sublayers 68A-68C comprise: a firstsublayer 68A formed of GaN, having a thickness of about 60 nanometers,and having a doping concentration of about 10¹⁸ dopants per cm³; asecond sublayer 68B formed of GaN, having a thickness of about 90nanometers, and having a doping concentration between 1.1 to 2 timeslarger than the doping concentration of the first sublayer 68A; and athird sublayer 68C formed of GaN, having a thickness of about 10nanometers, and having a doping concentration between 1.5 to 2.5 timeslarger than the doping concentration of the second sublayer 68B.However, it is understood that the thicknesses and doping concentrationsof this embodiment can vary by +/−fifty percent of the stated values.

It is understood that while heterostructure 60 is shown including ann-type side of the active structure 22 configured similar to theheterostructure 50 (FIG. 8), embodiments can include one or more of thep-type layers 62, 64, 66 located on the p-type side of the activestructure 22 in conjunction with any of the n-type side configurationsdescribed herein, including the heterostructures 40 (FIG. 7), 30 (FIG.6), 10 (FIG. 5). In each case, one or more of the p-type layers 62, 64,66 described herein can be epitaxially grown over the correspondingactive structure 22 of the heterostructure.

As described herein, the various heterostructures can be utilized tofabricate any of various types of optoelectronic devices. In anillustrative embodiment, a heterostructure described herein is utilizedin fabricating a light emitting diode. In a more particular illustrativeembodiment, the light emitting diode has a flip chip arrangement. Tothis extent, FIG. 10 shows a schematic structure of an illustrative flipchip light emitting diode 100 according to an embodiment. In this case,the diode 100 includes the heterostructure 10 (FIG. 5) on which anelectron blocking layer 62 and a p-type layer 66 (e.g., a claddinglayer) are formed. However, it is understood that this structure is onlyillustrative of the various heterostructures described herein.

As shown in conjunction with the device 100, a p-type metal 70 can beattached to the p-type layer 66 and a p-type contact (electrode) 72 canbe attached to the p-type metal 70. Similarly, an n-type metal 74 can beattached to the n-type layer 20 and an n-type contact (electrode) 76 canbe attached to the n-type metal 74. A surface of the n-type layer 20 canbe accessed using any solution, such as etching. The p-type metal 70 andthe n-type metal 74 can form ohmic contacts to the corresponding layers66, 20, respectively. In an embodiment, the p-type metal 70 and then-type metal 74 each comprise several conductive and reflective metallayers, while the n-type contact 76 and the p-type contact 72 eachcomprise highly conductive metal. In a further embodiment, the p-typelayer 66 and/or the p-type contact 70 can be transparent to theelectromagnetic radiation generated by the active structure 22. Forexample, the p-type layer 66 and/or the p-type contact 72 can comprise ashort period superlattice lattice structure, such as an at leastpartially transparent magnesium (Mg)-doped AlGaN/AlGaN short periodsuperlattice structure (SPSL). Furthermore, the p-type contact 72 and/orthe n-type contact 76 can be reflective of the electromagnetic radiationgenerated by the active structure 22. In another embodiment, the n-typelayer 20 and/or the n-type contact 76 can be formed of a short periodsuperlattice, such as an AlGaN SPSL, which is transparent to theelectromagnetic radiation generated by the active structure 22.

As further shown with respect to the optoelectronic device 100, thedevice 100 can be mounted to a submount 82 via the contacts 72, 76 in aflip chip configuration. In this case, the substrate 12 is located onthe top of the optoelectronic device 100. To this extent, the p-typecontact 72 and the n-type contact 76 can both be attached to a submount82 via contact pads 78, 80, respectively. The submount 82 can be formedof aluminum nitride (AlN), silicon carbide (SiC), and/or the like.

It is understood that the layer configuration of the optoelectronicdevice 100 described herein is only illustrative. To this extent, aheterostructure for an optoelectronic device can include an alternativelayer configuration (such as an alternative heterostructure describedherein), one or more additional layers, and/or the like. As a result,while the various layers are shown immediately adjacent to one another(e.g., contacting one another), it is understood that one or moreintermediate layers can be present in a heterostructure for anoptoelectronic device. For example, a heterostructure for anoptoelectronic device can include a Distributive Bragg Reflector (DBR)structure, which can be configured to reflect light of particularwavelength(s), such as those emitted by the active structure 22, therebyenhancing the output power of the device/heterostructure. Such a DBRstructure can be located, for example, between the p-type layer 66 andthe active structure 22.

In one embodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more devices designed andfabricated as described herein. To this extent, FIG. 11 shows anillustrative flow diagram for fabricating a circuit 126 according to anembodiment. Initially, a user can utilize a device design system 110 togenerate a device design 112 for a semiconductor device as describedherein. The device design 112 can comprise program code, which can beused by a device fabrication system 114 to generate a set of physicaldevices 116 according to the features defined by the device design 112.Similarly, the device design 112 can be provided to a circuit designsystem 120 (e.g., as an available component for use in circuits), whicha user can utilize to generate a circuit design 122 (e.g., by connectingone or more inputs and outputs to various devices included in acircuit). The circuit design 122 can comprise program code that includesa device designed as described herein. In any event, the circuit design122 and/or one or more physical devices 116 can be provided to a circuitfabrication system 124, which can generate a physical circuit 126according to the circuit design 122. The physical circuit 126 caninclude one or more devices 116 designed as described herein.

In another embodiment, the invention provides a device design system 110for designing and/or a device fabrication system 114 for fabricating asemiconductor device 116 as described herein. In this case, the system110, 114 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thesemiconductor device 116 as described herein. Similarly, an embodimentof the invention provides a circuit design system 120 for designingand/or a circuit fabrication system 124 for fabricating a circuit 126that includes at least one device 116 designed and/or fabricated asdescribed herein. In this case, the system 120, 124 can comprise ageneral purpose computing device, which is programmed to implement amethod of designing and/or fabricating the circuit 126 including atleast one semiconductor device 116 as described herein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 110 to generate thedevice design 112 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a stored copy of the program code can beperceived, reproduced, or otherwise communicated by a computing device.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 110 for designing and/or a devicefabrication system 114 for fabricating a semiconductor device asdescribed herein. In this case, a computer system can be obtained (e.g.,created, maintained, made available, etc.) and one or more componentsfor performing a process described herein can be obtained (e.g.,created, purchased, used, modified, etc.) and deployed to the computersystem. To this extent, the deployment can comprise one or more of: (1)installing program code on a computing device; (2) adding one or morecomputing and/or I/O devices to the computer system; (3) incorporatingand/or modifying the computer system to enable it to perform a processdescribed herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A heterostructure comprising: a substrate; an AlNbuffer layer located on the substrate; aAl_(x)Ga_(1-x)N/Al_(x′)Ga_(1-x′)N first superlattice structure locatedon the buffer layer, wherein 0.6<x≦1, 0.1<x′<0.9, and x>x′, and whereineach layer in the first superlattice structure has a thickness less thanor equal to one hundred nanometers; a Al_(y)Ga_(1-y)N/Al_(y′)Ga_(1-y′)Nsecond superlattice structure located on the first superlatticestructure, wherein y′<x′, 0.6<y≦1, 0.1<y′<0.8, and y>y′, and whereineach layer in the second superlattice structure has a thickness lessthan one hundred nanometers; an Al_(z)Ga_(1-z)N n-type layer located onthe second superlattice structure, wherein 0.1<z<0.85 and z<y′; and anAl_(b)Ga_(1-b)N/Al_(q)Ga_(1-q)N active structure, wherein b−q>0.05. 2.The heterostructure of claim 1, wherein the active structure isconfigured to emit electromagnetic radiation having a peak emissionwavelength between 300 nanometers and 360 nanometers, and wherein0.1<x′<0.8, 0.1<y′<0.65, 0.1<z<0.6, and 0<q<0.35.
 3. The heterostructureof claim 1, wherein the active structure is configured to emitelectromagnetic radiation having a peak emission wavelength between 260nanometers and 300 nanometers, and wherein 0.6<x′<0.9, 0.5<y′<0.8,0.4<z<0.75, and 0.2<q<0.6.
 4. The heterostructure of claim 1, whereinthe active structure is configured to emit electromagnetic radiationhaving a peak emission wavelength between 230 nanometers and 260nanometers, and wherein 0.65<z<0.85 and 0.45<q<0.75.
 5. Theheterostructure of claim 1, wherein at least one of x or y equals one.6. The heterostructure of claim 1, further comprising: anAl_(B)Ga_(1-B)N electron blocking layer located on the active structure,wherein B is at least 1.05*b; and a p-type GaN layer located on theelectron blocking layer.
 7. The heterostructure of claim 6, furthercomprising a graded p-type layer located between the electron blockinglayer and the GaN layer, wherein the graded p-type layer has an aluminummolar fraction that decreases from B at a heterointerface between theelectron blocking layer and the graded p-type layer to zero at aheterointerface between the graded p-type layer and the GaN layer. 8.The heterostructure of claim 6, wherein the GaN layer includes threesublayers, and wherein each sublayer has a doping concentration thatdiffers from an immediately adjacent sublayer by at least ten percent.9. The heterostructure of claim 1, further comprising a gradingstructure located between the second superlattice structure and then-type layer, wherein the grading structure has an aluminum molarfraction that decreases from y′ at a heterointerface between the secondsuperlattice structure and the grading structure to z at aheterointerface between the grading structure and the n-type layer. 10.The heterostructure of claim 1, further comprising a tensile/compressivesuperlattice located between the second superlattice structure and then-type layer.
 11. The heterostructure of claim 1, wherein the activestructure is configured to emit electromagnetic radiation having a peakemission wavelength between 300 nanometers and 360 nanometers, whereinthe n-type layer includes four sublayers, and wherein each sublayerdiffers from an immediately adjacent sublayer by at least one of: dopingconcentration or aluminum molar fraction.
 12. The heterostructure ofclaim 1, wherein the active structure is configured to emitelectromagnetic radiation having a peak emission wavelength between 260nanometers and 300 nanometers, wherein the n-type layer includes fivesublayers, and wherein each sublayer differs from an immediatelyadjacent sublayer by at least one of: doping concentration or aluminummolar fraction.
 13. A heterostructure comprising: a substrate; a bufferlayer located on the substrate, wherein the buffer layer is formed of agroup III nitride material including aluminum; a grading structurelocated on the buffer layer, wherein the grading structure is formed ofa group III nitride material having an aluminum molar fraction thatdecreases from an aluminum molar fraction at a bottom heterointerface toan aluminum molar fraction at a top heterointerface; a n-type layerlocated on the grading structure, wherein the n-type layer is formed ofa group III nitride material including aluminum having a molar fractionz, and wherein 0.1<z<0.85; an active structure including quantum wellsand barriers, wherein the quantum wells are formed of a group IIInitride material including aluminum having a molar fraction q and thebarriers are formed of a group III nitride material including aluminumhaving a molar fraction b, and wherein b−q>0.05; an electron blockinglayer located on the active structure, wherein the electron blockinglayer is formed of a group III nitride material including aluminumhaving a molar fraction B, and wherein B is at least 1.05*b; a p-typeGaN layer located on the electron blocking layer; and a graded p-typelayer located between the electron blocking layer and the GaN layer,wherein the graded p-type layer has an aluminum molar fraction thatdecreases from B at a heterointerface between the electron blockinglayer and the graded p-type layer to zero at a heterointerface betweenthe graded p-type layer and the GaN layer.
 14. The heterostructure ofclaim 13, wherein the active structure is configured to emitelectromagnetic radiation having a peak emission wavelength between 230nanometers and 260 nanometers, and wherein 0.65<z<0.85 and 0.45<q<0.75.15. The heterostructure of claim 13, further comprising: a firstsuperlattice structure located between the buffer layer and the gradingstructure, wherein the first superlattice structure is formed of aplurality of periods, each period including two layers formed of groupIII nitride materials including aluminum and having molar fractions xand x′, where x>x′; and a second superlattice structure located betweenthe first superlattice structure and the graded structure, wherein thesecond superlattice structure is formed of a plurality of periods, eachperiod including two layers formed of group III nitride materialsincluding aluminum and having molar fractions y and y′, where y>y′. 16.The heterostructure of claim 13, wherein the active structure isconfigured to emit electromagnetic radiation having a peak emissionwavelength between 300 nanometers and 360 nanometers, and wherein0.1<x′<0.8, 0.1<y′<0.65, 0.1<z<0.6, and 0<q<0.35.
 17. Theheterostructure of claim 13, wherein the active structure is configuredto emit electromagnetic radiation having a peak emission wavelengthbetween 260 nanometers and 300 nanometers, and wherein 0.6<x′<0.9,0.5<y′<0.8, 0.4<z<0.75, and 0.2<q<0.6.
 18. A method of fabricating adevice, the method comprising: creating a device design for the device,wherein the creating includes configuring a n-type side of aheterostructure for the device based on an active structure in theheterostructure including quantum wells and barriers based on a targetwavelength for the device, wherein the quantum wells are formed of agroup III nitride material including aluminum having a molar fraction qand the barriers are formed of a group III nitride material includingaluminum having a molar fraction b, and wherein b−q>0.05, wherein theconfiguring includes: configuring a grading structure located betweenthe active structure and a buffer layer of the heterostructure, whereinthe grading structure is formed of a group III nitride material havingan aluminum molar fraction that decreases from an aluminum molarfraction at a bottom heterointerface to an aluminum molar fraction at atop heterointerface; and configuring a n-type layer located between thegrading structure and the active structure, wherein the n-type layer isformed of a group III nitride material including aluminum having a molarfraction z selected based on at least one of: b or q; and fabricatingthe device according to the device design.
 19. The method of claim 18,wherein the configuring further includes: configuring a firstsuperlattice structure located between the buffer layer and the gradingstructure, wherein the first superlattice structure is formed of aplurality of periods, each period including two layers formed of groupIII nitride materials including aluminum and having molar fractions xand x′, where x>x′; and configuring a second superlattice structurelocated between the first superlattice structure and the gradedstructure, wherein the second superlattice structure is formed of aplurality of periods, each period including two layers formed of groupIII nitride materials including aluminum and having molar fractions yand y′, where y>y′, and wherein y′ and x′ are selected such thatz<y′<x′.
 20. The method of claim 18, wherein the creating furtherincludes configuring a p-type side of the heterostructure for the devicebased on the active structure.